AttributeError: __exit__ error on custom operator
focus on the code around the error:

import bpy

files = ['/home/zeffii/GL_fillet.mpeg', '/home/zeffii/GL_fillet2.mpeg']

for filepath in files:
    with bpy.data.movieclips.load(filepath) as (data_from, data_to):
        print(data_from, data_to)


Even from that snippet I get AttributeError: __exit__ 

I may be wrong, but this suggests to me that there is no context manager (in the python sense, not in the bpy.context sense) to use with x.y.z as (a, b) on the bpy.data.movieclips.load() operator. Hence that error. The documentation confirms this. movieclips.load() does return an object though!

If you just want to get stuff done, rewrite it so it doesn't use the with x.y.z as (a,b) context, instead assign to the left.

import bpy

files = ['/home/zeffii/GL_fillet.mpeg', '/home/zeffii/GL_fillet2.mpeg']

for filepath in files:
    clip = bpy.data.movieclips.load(filepath)
    # you know what you need here.
    print(clip.name)
    print(clip.size[:])
    print(clip.source)

&gt;&gt;&gt; GL_fillet.mpeg
&gt;&gt;&gt; (1280, 720)
&gt;&gt;&gt; MOVIE
&gt;&gt;&gt; GL_fillet2.mpeg
&gt;&gt;&gt; (1280, 720)
&gt;&gt;&gt; MOVIE


Additionally, there's also a typo I think. The original code has 

for name in eval("data_from." + datablock):
    # blah..


and you omit a necessary . after data_from..

Your scan_files function for movie_clips might start to look something like:

def scan_files(folderpath, datablock):
    exec("bpy.context.scene." + datablock + "_directory_list.clear()")  
    exec("bpy.context.scene." + datablock + "_directory_index = 0")  

    folderpath = bpy.path.abspath(folderpath)

    for _file in os.listdir(folderpath):
        filepath = folderpath + "\\" + file
        clip = bpy.data.movieclips.load(filepath)

        data_from = ...  # clip.name ?
        data_to = ... # clip.source ?

        for name in eval("data_from." + datablock):
            my_item = eval("bpy.context.scene" + datablock + "_directory_list.add()")
            my_item.name = name  # these two correspond to custom props group
            my_item.file = _file


Without seeing more of your code (got a github repository?) I'm not able to say more. Looks like an interesting project though.